This application is directed to improvements in a thread tapping screw, and more particularly to a novel and improved thread profile design for a thread tapping fastener. The novel thread profile in accordance with the invention is such as to improve the loading of the assembled threaded fastener and mating threads formed thereby upon driving into a workpiece so as to improve the loading that the resultant assembly can develop over that produced by other known thread forms.
Generally speaking, most male threaded fasteners which are used to effect a joint in relatively softer materials are of the self-threading or thread-forming type. Such fasteners are used, for example, in pliable plastic materials or in light alloy materials or nuts. These fasteners use the principal of plastic deformation of materials to produce, in the nut anchor or receiving workpiece pilot hole, a thread which mates with the male fastener thread. Generally speaking, thread designs used in these applications are of the spaced thread type, and are designed to give a good relationship between the tapping and stripping torques. Also of concern, particularly in these pliable plastic or light alloy materials is the hoop stress, or generally radially directed stress forces which are induced in the material by the formation of the thread due to driving of a self-threading or thread-forming fastener into the material.
Such pliable materials do not have the cold flow characteristics of harder materials such as steel, for example, and are in effect notch sensitive. By notch sensitive is meant the stress concentration which is found around the grooves or other discontinuities in a piece when stress or stretching forces are applies to the material. Plastics vary greatly in this regard, some being extremely pliable and others very brittle, and hence more notch sensitive. In any event, high stress concentrations applied during driving of a self-threading or thread-forming fastener tend to induce nut member or boss cracking which can compromise the effectiveness of the final joined assembly.
The term "boss", as used herein, refers to the general practice in plastic or other light or pliable materials to form an embossment or boss in the material about the area where a pilot hole is to be formed. Certain standards govern boss wall thickness relative to the size of the pilot hole and/or the nominal diameter of the fastener to be assembled therewith. Hence much of the so-called hoop stress applied to the material is in the boss wall thus defined. Any damage or cracking to the boss wall will, of course, detract from the effectiveness of the finished assembly. Finally, it is desirable in such assemblies to maintain the finished joined assembly in a condition of equilibrium even under external forces that may be applied to the jointed structure, such that the fastener and workpiece will not become loosened due to the effects of external forces applied thereto in service.